Examples of enzyme sensors known in the art are glucose sensors, urea sensors and uric acid sensors. These sensors are for measuring the concentration of a biological substrate after the concentration of hydrogen peroxide (H.sub.2 O.sub.2) produced by an enzyme reactor or the concentration of oxygen (O.sub.2) consumed by the reaction is measured amperometrically using an oxygen sensor or hydrogen peroxide sensor. For this reason, enzyme sensors generally are difficult to miniaturize. Another drawback is that these sensors cannot be utilized in enzyme reactions that are not accompanied by the consumption of oxygen or the production of hydrogen peroxide. One expedient for solving these problems is a sensor which determines the concentration of a biological substrate by measuring a change in pH that accompanies the progress of an enzyme reaction.
In recent years attempts have been made to fabricate miniature enzyme sensors by utilizing ISFET (ion-selective field-effect transistor)-type pH sensors. However, the adhesion between an enzyme film and the surface of a gate insulating film (such as a film of Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5) used in these ISFET's is poor. As a result, these sensors exhibit a somewhat low sensitivity, a large amount of drift and possess but a short service life.